WASHINGTON — Older women with mild memory impairment worsened about twice as fast as men, researchers reported Tuesday, part of an effort to unravel why women are especially hard-hit by Alzheimer’s.

Nearly two-thirds of Americans with Alzheimer’s are women.

At age 65, seemingly healthy women have about a one in six chance of developing Alzheimer’s during the rest of their lives, compared with a one in 11 chance for men. Scientists once thought the disparity was because women tend to live longer — but there’s increasing agreement that something else is responsible.

“Women are really at the epicenter of the Alzheimer’s disease crisis,” said Dr. Kristine Yaffe of the University of California-San Francisco. “We don’t really understand what this is all about.”

A series of studies presented Tuesday at the Alzheimer’s Association International Conference uncovered signs of that vulnerability well before symptoms hit.

First, Duke University researchers compared nearly 400 men and women with mild cognitive impairment, early memory changes that don’t interfere with everyday activities but that mark an increased risk for developing Alzheimer’s. They measured these people’s cognitive abilities over an average of four years and as long as eight years for some participants.

The men’s scores on an in-depth test of memory and thinking skills declined a point a year while women’s scores dropped by two points a year.

Age, education levels and even whether people carried the ApoE-4 gene that increases the risk of late-in-life Alzheimer’s couldn’t account for the difference, said Duke medical student Katherine Lin, who cowrote the study with Duke psychiatry professor Dr. P. Murali Doraiswamy.